The present application is based on Patent Application No. 11-220970 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a laser irradiation optical system, and specifically relates to a laser irradiation optical system which divides a laser beam, shapes each laser beam after division, and emits each laser beam after shaping.
2. Description of the Prior Art
A characteristic of a laser beam is the ability to increase intensity and reduce the beam width, for diversified fine processing of an object surface. In recent years, increased processing efficiency has been achieved by performing the same process at a plurality of parts by dividing a laser beam emitted from a laser light source into a plurality of laser beams, and irradiating an object with the laser beams after division of the laser beam.
It is necessary to equalize the intensity of the laser beams after division because the different parts of the object cannot be processed equally due to differences in intensity between the emitted laser beams. The intensity distribution of each laser beam must be set in accordance with shape of the object being processed because the object is processed in a shape corresponding to the intensity distribution of the laser beam. For example, when forming a hole having a square cross section and a fixed depth in an object, the contour of the cross section perpendicular to the optical path must be rectangular, and the laser beam must have a uniform intensity distribution within this cross section.
Accordingly, the laser irradiation optical system used for such purpose not only simply divides the laser beam emitted from the light source, but also must equalize the intensity of all laser beams after division to attain a desired intensity distribution of each laser beam in a cross section parallel to the optical path and in a cross section perpendicular to the optical path. In general, the intensity distribution in a cross section perpendicular to the optical path of the laser beam emitted from a light source is a Gaussian type distribution, and is not suitable to be used directly in most cases. For this reason, the intensity distribution of the laser beam is converted, i.e., shaped, by the laser irradiation optical system.
The construction of a conventional laser irradiation optical system is briefly shown in FIG. 15. This laser irradiation optical system 7 divides a laser beam L1 emitted from a laser light source 71 into four laser beams L2, and equalizes the intensity distribution of each laser beam L2 in a cross section perpendicular to the optical path. Three half-mirrors 73 are provided to divide the laser beam L1, and the transmittance and reflectivity rate of the three half-mirrors 73 are set so as to equalize the intensity of the laser beams L2 after division.
The laser means L2 after division are directed to a light shield 77 provided with openings 77a having identical size and shape and disposed at equal distances via the total reflecting mirror 74. The light shield 77 transmits only the center part of the laser beam L2 having a Gaussian distribution so as to render a uniform intensity distribution in a cross section perpendicular to the optical path, and regulates the shape of the contour of this cross section via the shape of the opening 77a. 
A beam expander 72 for broadening the beam width of the laser beam L1 is arranged on the optical path from the light source 71 to the half-mirror 73, and a lens 75 for converging the laser beam L2 on the light shield 77 is arranged on the optical path of each divided laser beam L2. The four lenses 75 have identical performance, and the distance from each lens 75 to the light shield 77 is equal. A reducing optical system 78 is provided on the optical path of the laser beam L2 transmitted through the opening 77a, and each laser beam L2 is condensed in beam width and mutual spacing via the reducing optical system 78 and irradiates the irradiation object surface S.
In laser irradiation device 7, the divided laser beams L2 are shaped by the opening 77a of the light shield 77, the lens 75, and the reducing optical system 78. The main element among the aforesaid elements fulfilling this function is the opening 77a which regulates the intensity distribution in a direction perpendicular to the optical path. The condition of the shaping of the divided laser beam L2 by the opening 77a of the light shield 77 is shown schematically in FIG. 16.
In this laser irradiation device, the division of the laser beam emitted from the light source occurs in several stages. For this reason, the overall structure is enlarged, there are many optical elements, and the relative positions of the elements cannot be easily set. This problem becomes pronounced as more laser beams are produced after division.
The divided laser beams have a Gaussian type distribution similar to the laser beam before division, and the range in which the intensities are near fixed is narrow. Accordingly, the majority of the laser beam is eliminated in shaping, such that there is poor usage efficiency of the laser beam from the light source. In the example of FIG. 16, only one half of the laser emitted from the light source is used.
Laser beam division also can be accomplished by a diffraction element; when such an element is used, the beam can be divided once, and the overall structure can be expected to be made more compact. However, there is no example of use of a diffraction element for division in laser irradiation optical systems which shape the beam after dividing the beam. This is because the laser beams are overlap directly after division, and the shaping of each laser beam is difficult in this state, the direction of travel of each laser beam differs after division, and setting the conditions for managing the direction of travel and the conditions for shaping are difficult.
An object of the present invention is to improve a compact laser irradiation optical system for dividing and shaping a laser beam into a plurality of laser beams and emitting the laser beams after shaping.
A particular object of the present invention is to provide a laser irradiation optical system which has little loss of the laser when shaping.
These objects are attained by a laser irradiation optical system having the following construction.
A laser irradiation optical system comprising a dividing means for dividing a first entering laser beam once, and producing a plurality of second laser beams having beam widths equal to the beam which of the first laser beam and advancing in mutually different directions; a condensing means for condensing each second laser beam to mutually advance in near the same direction; and a shaping means for converting the intensity distribution of a cross section perpendicular to the optical path of each second laser beam in the optical paths of the mutually separated second laser beams.
The division means divides the entering first laser beam into a plurality of second laser beams, but since this division occurs only once, the overall construction of the laser irradiation optical system is compact. The division means may divide the first laser beams in only one direction, or may divide the first laser beam in two directions. All the second laser beams obtained by division have approximately equal beam widths. Since it is also possible for the division means to approximately equalize the intensities of all the second laser beams, all the second laser beams can be rendered equivalent excluding the different directions of travel.
The second laser beams are set to advance in approximately the same direction via the condensing means. Accordingly, the second laser beams can irradiate the irradiation object from identical directions. Moreover, the condensing means produces a condensed beam of each second laser beam, such that the beam width of each second laser beam can be easily be extremely reduced on the object surface.
Although the second laser beams overlap directly after division, they mutually separate by traveling a certain distance. The condensing means may be disposed on the optical path of the overlapping second laser beams, or may be disposed on the optical path after separation. In the former disposition, the second laser beams are reliably separated so as to advance in mutual parallel because the beam widths are narrowed when the beams are condensed by the condensing means. Furthermore, the position of separation in this instance is rendered nearer to the dividing means than when a condensing means is not used.
Each second laser beam has its intensity distribution converted in a cross section perpendicular to the optical path by the shaping means. That is, the shaping means shapes the second laser beams in a direction perpendicular to the optical path. Since this shaping is accomplished in the optical path in which the second laser beams are mutually separated, the second laser beams can be set in a desired shape easily and reliably.
When the condensing means is disposed at a position at which the second laser beams overlap, the shaping means is disposed in the optical path of the second laser beams that have passed through the condensing means. When the condensing means is disposed at a position at which the second laser beams are separated, the shaping means can be disposed either in the optical path of the second laser beams that have reached the condensing means, or in the optical path of the second laser beams that have passed through the condensing means.
In this laser irradiation optical system, the dividing means is an element which divides a first laser beam by diffraction to produce a second laser beam, and the condensing means is an element having a focal point on or near the dividing means, and arranged in the optical path anterior to the position of mutual separation of the second laser beams, and which satisfies the relationship of equation 1.
fxc2x7{1xe2x88x92mxc2x7xcexxc2x7f/(pxc2x7W)}xe2x89xa6Zdxe2x89xa6fxc2x7{1+mxc2x7xcexxc2x7f/(pxc2x7W)}xe2x80x83xe2x80x83Eq. 1
Where xcex represents the wavelength of the first and second laser beams, f represents the focal length of the condensing means, m represents the minimum value of the absolute value of the difference of the degree of diffraction of the second laser beams in the same diffraction direction, W represents the beam width of the first laser beam in this diffraction direction, p represents the sequence period of the grating unit of the dividing means in this diffraction direction, and Zd represents the distance from the condensing means to the shaping means with the advancing direction of the second laser beam designated positive.
The left side of equation 1 represents the position at which the two second laser beams having the smallest difference in degree of diffraction are mutually separated as condensed beams by the condensing means; at this position all of the second laser beams are separated. The right side of equation 1 represents the position at which the two second laser beams having the smallest difference in degree of diffraction again overlap as divergent beams after being once condensed; overlap of all the second laser beams occurs after this position.
Accordingly, the shaping means which has a relative position to the condensing means stipulated by equation 1 is reliably positioned in the optical path where the second laser beams are mutually separated. When the disposition of the condensing means is at a position at which the two second laser beams having the smallest difference in degree of diffraction are mutually separated, the left side of equation 1 becomes [0], and when the condensing means is disposed even nearer to the dividing means, the left side of equation 1 becomes a positive value.
The dividing means is an element which divides the first laser beam by diffraction to produce second laser beams, and the condensing means is an element having a focal point on or near the dividing means and arranged in the optical path posterior to the position of mutual separation of the second laser beams, and which satisfies the relationship of equation 2.
xe2x80x83fxc2x7{pxc2x7W/(mxc2x7xcexxc2x7f)xe2x88x921}xe2x89xa6Zdxe2x89xa6fxc2x7{1+mxc2x7xcexxc2x7f/(pxc2x7W)}xe2x80x83xe2x80x83Eq. 2
The definition of the various symbols in equation 2 are the same as described previously. The left side of equation 2 represents the position at which the two second laser beams having the smallest difference in degree of diffraction are mutually separated while maintaining the path after diffraction; at this position all of the second laser beams are separated. The right side of equation 2 represents the position at which the two second laser beams having the smallest difference in degree of diffraction again overlap as divergent beams after once being condensed by the condensing means; the overlap of all the second laser beams occurs after this position.
Accordingly, the shaping means which has a relative position to the condensing means stipulated by equation 2 is reliably positioned in the optical path where the second laser beams are mutually separated. Under the condition that the disposition of the condensing means is at a position at which the two second laser beams having the smallest difference in degree of diffraction are mutually separated, the left side of equation 2 becomes not a value of zero or greater, but a negative value. The fact that Zd is negative means that the shaping means is disposed between the dividing means and the condensing means.
When the dividing means is a diffraction element that divides the first laser beam in one direction, the element generates a unidimensional diffraction, whereas when such an element divides the first laser beam in two directions, the element generates bidimensional diffraction. In the case of bidimensional diffraction, equation 1 or equation 2 obtain for both diffraction directions.
The condensing means and the shaping means may be formed as a single integrated element. Such an element can be near the condensing means and shaping means, i.e., can be used when Zd is approximately equal to [0] on the left side of equation 1, and when Zd of equation 2 is approximately [0].
The shaping means may be a single element having a plurality of parts for converting the intensity distribution of the second laser beams. In this case, the shaping means positioning may easily match the dividing means and the condensing means.
The shaping means may be an element for converting the intensity distribution of the second laser beams by diffraction. The conversion of the intensity distribution by a diffraction element allows great freedom, and the second laser beam can easily achieve a desired shape by the setting sequence of the diffraction grating. Furthermore, no part of the entering laser beam is eliminated, and all parts can be used in the conversion, thereby increasing the laser usage efficiency. A diffraction element used as a shaping means may be a binary type provided with small surfaces at different heights in steps, or may be a free curvature type provided with moderately ranged staged surfaces.